//a*x=base+a*z
//b*y=base+b*z
//z=(b*y-a*x)/(b-a)
#include<iostream>
using namespace std;
int main()
{
  double x,a,y,b;
  cin>>x>>a>>y>>b;
  cout.precision(2);
  cout<<fixed;
  cout<<(b*y-a*x)/(b-a);
}